Low Pressure Casting Porosity: Causes, Controls and Buyer Checks

If you’re sourcing aluminum parts, low pressure casting is probably on your shortlist. The process fills the die from below in a controlled manner, which reduces turbulence and gives better internal quality than conventional gravity pouring. But “better” is not “perfect.” Low pressure casting porosity still happens, and when it does, it costs you money. The classic finding is a housing or valve body that passes dimensional inspection at the foundry and then leaks in your pressure test bay. Here’s what causes that porosity, what a real casting supplier should be doing about it, and the checks you should run before placing an order.

What causes porosity in low pressure casting

Porosity in aluminum castings comes in two families: gas porosity and shrinkage porosity. They look similar on an X-ray and on a machined surface, but they form for different reasons.

Gas porosity starts in the melt. Hydrogen dissolves easily in liquid aluminum, then falls out of solution as the metal solidifies. The result is a fine set of pores distributed through the casting. Where does the hydrogen come from? Humid shop air, wet fluxes, damp tools, even surface moisture on the charge material. A foundry that doesn’t degas the melt and control temperature will produce porous parts, and low pressure casting does not fix a dirty melt.

You can also trap gas mechanically. If the die fills too fast, the metal folds air into itself. Vents that are too small, blocked vent lines, or the wrong die coating all contribute.

Shrinkage porosity is the other half. Aluminum shrinks as it solidifies. The last liquid inside a thick section has to feed the metal already solidifying around it. If that supply is cut off, you get a void. In low pressure casting, air pressure on the melt surface pushes metal up into the die, and holding that pressure while the casting solidifies feeds the shrinkage. That’s the whole point of the process. But if the pressure is released too early, or a thin gate freezes before the thick section is done feeding, shrinkage porosity shows up exactly where you don’t want it.

The honest summary: low pressure casting reduces the turbulence-related defects that plague gravity pouring. It doesn’t automatically remove gas and shrinkage porosity. Those depend on melt discipline and process control.

Why porosity gets expensive downstream

Porosity is a late-failure defect. It’s subsurface, so you can’t see it on an incoming visual inspection. The part looks fine, dimensions are met, and then the problem appears in your own operation.

First stop: machining. When the cutter breaks into a pore, you get a bad surface, sometimes a rejected part, and always an interruption. If porosity is clustered, you can lose multiple parts from the same batch.

Second stop: pressure testing. For valve bodies, pump housings, and anything that holds fluid or air, a leak is a hard reject. The part was cast, machined, and possibly coated before the leak showed up. That’s expensive scrap — and worse when the leak shows up in your customer’s test bay, not yours.

Third stop: service life. Pores act as stress concentrators. Under cyclic loading, a porous region can start a fatigue crack long before a sound casting would fail. You rarely see that on a drawing, but it decides how long the part survives in the field.

None of this is visible from the outside. That’s why porosity has to be controlled at the foundry, not caught at your receiving dock.

Process controls that separate a real foundry from a parts broker

Porosity is not random. It’s the result of process decisions, and a serious supplier controls each one.

Melt preparation. The melt must be degassed to drive out hydrogen. Charge materials need to be clean and dry. Melt temperature has to stay in a tight window. You can’t see any of this from outside the plant, so you have to ask directly.

Pressure curve. Low pressure casting gives you a tool that gravity casting doesn’t: sustained pressure during solidification. The supplier should know exactly when to raise pressure, how long to hold it, and when to drop it. Release early, and shrinkage gets a free ticket.

Die thermal control. Uneven die temperature creates hot spots, and hot spots create shrinkage porosity. Die heating, cooling lines, and cycle timing all matter. If the foundry opens the die on a timer and logs temperatures, that’s a good sign. If they work from habit, you’re at risk.

Gating and venting review. Decisions made during tool construction determine how the die fills and feeds. A supplier that reviews gating and venting with the customer before cutting steel is a supplier that’s actually managing the process.

Documented parameters. Pressure hold time, die temperature range, melt temperature, cycle time — these should exist on a process sheet. If nobody can show you the numbers, there is no process, only a series of hopes.

Waking runs low pressure aluminum casting in-house, alongside gravity casting and investment casting. That matters to buyers: the same engineers who set the process also answer for the parts. There is no third party to blame.

Inspection: X-ray, metallurgy, leak testing, traceability

Internal porosity can only be caught two ways: nondestructive testing that looks inside the metal, or destructive sectioning. Know which one your supplier actually does, on what equipment, and how often.

X-ray NDT. X-ray is the primary screening tool for porosity and shrinkage. Waking operates an XTH320 X-ray system in-house for internal defect detection, so porosity screening doesn’t become a slow, expensive special order. When a supplier owns its X-ray equipment, the question changes from “can you X-ray?” to “which parts do you screen, and what do you do when you find something?”

Metallurgical analysis. Waking’s lab uses an inverted metallurgical microscope for material structure analysis. Porosity is often connected to the microstructure — how the aluminum solidified and whether the structure matches the grade. Lab equipment like this signals that a foundry treats defects as something to understand, not just something to return.

Leak testing. If you’re buying pressure-containing parts — and valve bodies belong in this group — leak testing at the foundry is worth more than a contract clause. Waking performs leak testing on pressure-containing parts, so a valve body can be verified before it ships, not just after you machine it.

Traceability. Suppose a batch fails at your end. Can the supplier tell you which castings are affected, which melt they came from, and what the inspection record said? Waking’s quality system keeps batch traces and issues material certificates with inspection reports. Traceability turns a field failure into a defined problem instead of an open-ended argument. The management system behind this is documented through ISO 9001 and IATF 16949 certification.

Dimensional inspection matters too — Waking uses COORD3 and ZEISS SPECTRUM CMMs — but don’t confuse dimensional accuracy with internal soundness. A perfect envelope can hide a porous core.

Buyer checks before you place the order

Here is what purchasing and engineering teams should put into the RFQ and the development phase:

• Define porosity limits on the drawing. Don’t write “no porosity.” It isn’t physically achievable, and it gives nobody a workable target. Agree on an acceptance level and state whether it applies to the whole part or only to critical zones.
• Mark the zones that matter. Show the foundry which areas will be machined, leak tested, or both. A good foundry will design gating and inspection around those exact areas.
• Ask for the inspection plan. Will X-ray screening be done on first articles? On which zones? Is leak testing done on all pressure parts, or on a sample?
• Use sample approval. The first article and sample approval stage is where porosity problems are cheapest to fix. Section a sample, machine it at your facility, and look at the cut surfaces.
• Confirm traceability. Ask how a part links back to its batch, melt, and inspection records, and what documents ship with the order.
• Check where machining happens. If the foundry machines in-house, it sees the surfaces where porosity reveals itself. Waking runs 400+ CNC machines inside its own facility, so machined surfaces get examined by the people who cast them. That’s a real inspection step, not a bureaucratic one.

Questions to ask your low pressure casting supplier

If you take one thing from this article, take this: porosity is controlled by process discipline, so interview the supplier like an engineer, not like a catalog shopper.

• How do you degas the melt, and how do you verify that it worked?
• What is your pressure holding phase, and what parameter tells you it’s working correctly?
• Who reviews gating and venting when a new die is built?
• Which parts get X-rayed, and which zones are screened?
• How do you leak test pressure-containing parts?
• What happens after a porosity rejection — root cause analysis, or just a re-cast?
• Can you trace a production batch back to its material and inspection records?

Porosity in low pressure casting is not a mystery. It’s gas, shrinkage, and process discipline — and the third one decides your reject rate. A foundry that controls its melt, manages its pressure curve, and inspects internally will give you parts that hold pressure. One that doesn’t will give you castings that look fine and test results that don’t. Ask the questions, check the equipment, and look at the cut surfaces. That’s how you buy low pressure castings that perform — not just ones that photograph well.

Frequently Asked Questions

How can I tell if porosity in a low pressure aluminum casting is gas porosity or shrinkage porosity on an X-ray or machined surface?

Gas porosity and shrinkage porosity look similar on an X-ray and on a machined surface, but they form differently. Gas porosity comes from dissolved hydrogen or mechanically trapped air and often appears as fine distributed pores, while shrinkage porosity occurs in thick sections where the last liquid cannot feed solidification. A metallurgical analysis of the microstructure can help connect the porosity to how the aluminum solidified.

What melt preparation steps should I require from a low pressure casting foundry to avoid hydrogen gas porosity?

The foundry should degas the melt to drive out hydrogen, keep charge materials clean and dry, and hold melt temperature within a tight window. You should also ask whether they control shop humidity, flux moisture, and tool dryness, since any surface moisture on charge material can contribute hydrogen. These controls are not visible from outside the plant, so request documented melt preparation records.

Why does a low pressure cast part still fail a pressure test if the process is supposed to reduce turbulence?

Low pressure casting reduces turbulence-related air entrapment, but it does not automatically remove gas or shrinkage porosity. If the melt is not degassed or the pressure is released before thick sections finish feeding, subsurface porosity can remain and open into a leak path during machining or pressure testing. That is why leak testing and X-ray screening are important buyer checks.

What inspection evidence should a casting supplier provide for porosity control in low pressure aluminum parts?

Ask for X-ray NDT screening results, metallurgical analysis from a lab microscope, and leak testing records for pressure-containing parts. Waking, for example, runs an XTH320 X-ray system and an inverted metallurgical microscope in-house, and supports batch traceability with material certificates. Documented process parameters such as pressure hold time, die temperature, and melt temperature should also be available.

Key Parameters at a Glance